Relative to pitot tube icing and potential results, see Post #918, Pg.46, Post#1083 & 1084, Pg.55 both in the Tech Log, AF447 discussions.
Also refer to: EASA AD No.: 2010-0271 (22 December 2010) for modification of responses to erroneous air speeds.
As I interpret this AD, when at high altitude cruise in normal law, AP & AT on, the pitot tubes begin to ice leading to disagreement and the controls come out of normal law into Alt law causing the FD to disappear while disconnecting the AP & AT. But then, the icing becomes equalized in at least two pitots, indicating the return of FD and the ability to revert to AP & AT once again. If the AP & AD are immediately re-engaged, the air speed may be falsely high and a pitch up and N1 reduction may be called for by the AP & AT leaving the aircraft in an immediate and potentially stall condition, while leaving the flight crew in a situation where a stall warning and speed warning are received at the same time, confusion?
It is unclear how the AD came about, AF447 or prior upset events that were analyzed.